A woman is in custody after allegedly brandishing the head of a child near a metro station in Moscow on Monday. Photo: Youtube

According to witness reports, the unidentified woman pulled the severed head out of her bag shouting "I killed a child" and "I am a terrorist," RT news reported.

Russia's Interior Ministry confirmed that the woman had been holding a real human head of a child, thought to be about three or four years old.

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The woman, aged in her late 30s and originally from Uzbekistan, was walking around the entrance to the metro for about an hour. She appeared to be disoriented. She was wrestled to the ground by police.

Earlier on Sunday (Moscow local time), police said a child's headless body had been found in an apartment block near the same metro station.

Rescuers recovered the body of a child between three or four-years-old after they had extinguished a fire raging in an apartment block, Russia's Investigative Committee said in a statement.

The suspect waited until the child's parents left the apartment with an older child, then killed the child and set the apartment on fire, the Investigative Committee said in a Russian-language statement.

Officials said the woman was working as a nanny and allegedly murdered the child while her parents were not at home, according to the report.

"According to preliminary data, the child's nanny - a citizen of one of the Central Asian states, born in 1977 - waited until the parents left the apartment with their older child, [and] for unknown reasons, committed the murder of a minor, set fire to the apartment and left the scene," according to a English translation of the Investigative Committee statement.

The girl was identified as Nastya M., according to LifeNews.

A police report suggested the woman may have taken some form or narcotics prior to the incident, the news site reported.

The woman is being detained for a forensic psychiatric examination.

A criminal investigation is continuing.

Some rights activists warned of a possible backlash against migrant workers from Central Asia who have in the past been targeted for beatings by far-right groups.

Security services are on high alert for a possible terrorist attack after a Russian passenger plane was blown out of the sky above Egypt in October, killing all 224 people on board. Islamic State said it was responsible and had acted to avenge the Kremlin's decision to launch air strikes in Syria.